1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
In the field of wireless telecommunications, new multicast services like video streaming, game delivery or news clips are becoming more and more popular on mobile handsets and user equipment (UE) like Personal Digital Assistants (PDAs) and notebooks. Release 6 of the Third Generation (3G) Universal Mobile Telecommunications System (UMTS) identifies an implementation of Multimedia Broadcast/Multicast Services (MBMS) that has focused on the use of transport channels between the Base Station Router (BSR) or Node B and the UE, using the Forward Access CHannel (FACH), Dedicated Channel (DCH) and Donlink Shared Channel (DSCH). In Release 5 and above, High Speed DSCH (HS-DSCH) has been shown to provide high-speed downlink connections for unicast users. While the HS-DSCH allows High Speed Downlink Packet Access (HSDPA) to increase peak data rates and mean throughput per user based on individual information flows (e.g., unicasting), it does not address what is believed to be a major application in 3G services—multicasting.
The benefit on system performance and capacity by the use of HS-DSCH for multicasting may be less than desired due to the fact that this implementation would be used for a unicast-like scheme where users are being sent the same packet of data by transmitting it a number of times either in the code or in the time domain (code division multiplexing or time division multiplexing).
HS-DSCH is shown to be an efficient mechanism for multicast services. With HS-DSCH's Incremental Redundancy (IR) mechanism, code/modulation schemes are selected aggressively since more redundancy can be added when needed. Moreover, the HS-DSCH also uses adaptive coding and modulation, fast scheduling and Hybrid Acknowledgement ReQuest (H-ARQ) combining of retransmissions to improve the overall average throughput. It is believed that these technologies will improve the MBMS services over that given by current MBMS services based on Re199/4 and Re16 solutions.
However, when multicast services are using Re199 or HS-DSCH mechanisms, the worst user remains the bottleneck of the system. Currently, all Re1.99/4 systems conservatively select the coding and/or modulation and transmit power parameters to satisfy the worse case user. This limits the multicast service capabilities that could be provided because of the compromise that needs to be made between individual service provision, multicast service provision and the management of cell resources. Thus, the Re199 based system will fix the coding and power, while the HS-DSCH system would adapt those parameters based on the feedback coming from the worst user. In particular, as the selected Transport Format and Resource Combination (TFRC) is based on the worst Channel Quality Information (CQI), of all users in this group, then CQI is equivalent to a Signal to Noise Ratio (SNR) proportional to min{PL1.X1,K,PLN.XN}, where N is the number of users in the group, PL is the path loss and X is the probability distribution of the fast fading.
The mechanisms used in the 3GPP standard to support multicast to a group of users in the coverage area of a cell include:                The use of FACH;        The use of DCH; and        The use of MBMS.        
Supporting multicast via DCH is costly. The same multicast channel has to be replicated by the number of users using the multicast service. For a large multicast population in a cell this then is very costly in terms of cell resource utilization. DCH based multicasting effectively becomes a unicast service and if there are many users in a given cell that use the same multicast service, then it would rapidly consume all of the cells resources.
Supplying multicast services via the FACH is an improvement over unicasting. Principally, the FACH transmission parameters are set to cover users at the very edge of the cell and usually will yield low bit rates. This means that the FACH has relatively high power compared to other channels. Moreover, when a multicast service requiring high bit rate is used over the FACH it would consume vast amounts of cell resource. So, for low bit rate multicast services the FACH may have limited applicability, but for higher bit rate services it is not. While the actual bandwidth that can be transmitted by the FACH can be high (approaching 2 Mb/s), unfortunately this may not be practically achieved. For example, studies have shown that to supply a 128 kb/s Multicast channel over the entirety of a cell would take upwards of 70% of the available channel. Thus, providing MBMS over FACH is inefficient and has limited practical use.